1. Field of the Invention
The present invention relates generally to connecting the sides of two or more acoustic honeycomb sections together to form a spliced acoustic honeycomb. More particularly the invention is directed to providing improved seams between acoustic honeycomb sections, especially where multiple honeycomb sections are connected together to form a contoured or curved acoustic honeycomb structure.
The invention is particularly useful for seaming or splicing together curved acoustic honeycomb sections that are seamed together to form engine nacelles and other acoustic dampening structures.
2. Description of Related Art
It is widely recognized that the best way of dealing with excess noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic treatments to the structure at the noise source. One particularly problematic noise source is the jet engine used to propel aircraft Acoustic treatments are typically incorporated in the engine inlet, nacelle and exhaust structures. These acoustic treatments include acoustic resonators that contain relatively thin acoustic materials or grids that have millions of holes that create acoustic impedance to the sound energy generated by the engine. The basic problem that faces engineers is how to add these thin and flexible acoustic materials into the structural elements of the jet engine and surrounding nacelle to provide desired noise attenuation.
Honeycomb has been a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight and each honeycomb cell can be used as an acoustic resonator. For acoustic applications, the goal has been to somehow incorporate the thin acoustic materials into the honeycomb structure so that the honeycomb cells are closed or covered. The closing of the cells with acoustic material creates the acoustic impedance upon which the resonator is based.
Forming acoustic style honeycomb into complex curved structures required for engine nacelles is a major design consideration. It is very difficult to form a single honeycomb into an engine nacelle without significantly altering the honeycomb and its acoustic properties. Accordingly, numerous sections of curved or slightly curved acoustic honeycomb sections are typically spliced together to form the cylindrical nacelle structure.
The sides of the honeycomb that are spliced together are composed of numerous protruding cell walls that are typically referred to as “dog ears”. The protruding cell walls that make up the dog ears have ends that present a relatively small surface area for bonding of the two honeycomb sections together. In addition, it is very difficult to position the honeycomb sections so that the ends of the dog ears on opposing honeycomb sections are lined up and close enough for bonding.
The preceding bonding/splicing issues have been typically solved by filling the seam line, and the partial cells that border the seam line, with an adhesive. The adhesive is used to completely fill all of the open spaces along the seam line to provide a solid and secure bond. For aerospace applications, the adhesive is typically a foam-type adhesive to keep the weight of the seam at a minimum. An advantage of filling the seam line and surrounding partial cells with adhesive is that the surface area of the honeycomb which is available for bonding is much larger than the surface area provided by the ends of the dog ears. A strong bond is achieved due to the relatively large surface area of honeycomb that interacts with the adhesive to form the seam. In addition, a foam-type adhesive can be used to bond together honeycomb sides that are curved and/or serpentine. Since bonding is accomplished by simply filling the seam line with adhesive, the shape and orientation of the dog ears and partial cells along the seam line can vary without affecting the overall performance of the seam.
The use of an adhesive seam line does have disadvantages. For example, the acoustic properties of the honeycomb cells that are filled with adhesive are significantly altered or destroyed. Solid adhesive seams tend to be wide and relatively heavy, which is not desirable for aerospace applications where limiting weight is an important design consideration. Also, the strength of the seam can vary depending, upon the orientation of the dog ears on opposite sides of the seam line. In addition, the relatively wide adhesive seams tend to be stiff and may limit the ability to bend or otherwise shape the overall bonded honeycombs into a curved structure, such as an engine nacelle.
It would be desirable to provide a way of splicing or seaming honeycomb together that includes the advantages in bond strength provided by the solid adhesive seams described above while avoiding the disadvantages with respect to acoustic properties, seam weight and seam stiffness.